Coaxial terminal assembly



Sept. 24, 1946. w. w. RlGRoD ETAL COAXIAL TERMINAL ASSEMBLY Filed Augf 12, 1942 Patented Sept. 24, 1946 UNITED STATES PATENT OFFICE COAXIAL TERMINAL ASSEMBLY Application August 12, 1942, Serial No. 454,615

7 Claims. l

This invention relates to coaxial terminal assemblies, and while shown in direct connection with an ultra high frequency device, is of a character usable in any part of a coaxial line where a bead of dielectric is employed for spacing and/or sealing the channel enclosed by the tubular part of the coaxial line.

Presence of a dielectric bead in a coaxial line, and more especially in a terminal structure presents an impedance discontinuity. For instance, in association with an ultra high frequency generator sold under the trade name of Klystron, a bead oi' Corning '704 glass has been used at the end of a short coaxial line, said line constituting a plug-in terminal for attachment of a further coaxial line leading to some other instrumentality. In that particular construction the bead vacuum seals the parts of the coaxial line of the terminal so that one end is under the Vacuum of the generator whereas the otherl end of the terminal is open to the atmosphere. The speciiic construction referred to utilizes a bead of approximately one quarter inch in length and about the same or somewhat greater diameter. It has been determined that such a bead presents a reection factor of about due to the irnl pedance discontinuity, thereby seriously impairing the eiiiciency of the available output of the generator.

An object of the present invention is to overcome the high percent reection loss of a coaxial line bead.

A further object is to overcome the reflection loss of a vacuum-sealing bead in a coaxial line terminal.

Another object of the invention is to provide a structure which is readily manufactured, strongly built, conveniently assembled and applicable at a later time to the concentric line portion previously sealed to the generator.

Still further objects will appear to those skilled in the art both by direct reference thereto as the description proceeds and by implication from the context.

Referring to the accompanying drawing in which like numerals of reference indicate similar parts throughout the several views;

Figure l is a longitudinal sectional view of a concentric line terminal applied in place on an ultra high frequency generator device by way of illustration; and

Figure 2 is a cross-sectional view on line II-II of Fig. 1.

In the specic embodiment of the invention illustrated in said drawing, the reference numeral I!! designates generally the evacuated body of an ultra high frequency generator of which the above-mentioned Klystron is an example. From this generator body protrudes a hollow pipe or tube I I formingpart of a coaxial terminal. This tube II is preferably of a material having substantially the same coefficient of expansion as borosilicate glass, such material being sold in trade as Kovan One end of this Kovar tube is silver soldered or otherwise mounted vacuum tight on the body Il] and in communication with the interior of said body. The other end of said Kovar tube is sealed with a dielectric closure or bead I2, preferably of borosilicate glass of the character just mentioned. Said closure I2 is of a generally cylindrical formation the outer cylindrical surface by preference constituting a continuation of the outside cylindrical surface of the hollow pipe or tube II to which it is sealed,

Axially of the tube I I and closure I2 is the central member or rod I3, the assembly of these parts being known in the trade as a coaxial line. The inner end of the rod forms a suitable loop I4 Within the resonant cavity with the terminus of the rod silver soldered or otherwise joined to the inner terminus of the tube. The outer end of the central member or rod I3 continues forwardly through and beyond the dielectric closure or bead I2, said bead making sealing contact therewith. As shown, said closure or bead is formed with a body portion I5 which is cylindrical at its outer face in continuation of the cylindrical outer face of a collar portion I6 the far end of which is the part sealed to the metallic tube I I. The body portion of the bead I2 iills the entire cross-sectional area from the said outer cylindrical surface to the coaxial or central rod member I3, and it is this portion of the bead which heretofore has been troublesome in causing wave energy reflection.

In accordance with the present invention thisv impedance discontinuity may be overcome by using with the said bead I2 a washer-like ring I7 of dielectric engirdling the body portion I5 of said bead. This ring is perhaps best made of a material having a dielectric constant substantially equal to that of the bead, but from a practical constructional standpoint a substance the dielectric constant of which differs considerably from that of glass may be employed. We have found, for instance, that polystyrene recommends itself for use even though its dielectric constant at the wave length employed O\=l0 cm.) is only about half of the dielectric constant of 3 the particular glass (Corning '704) preferably used.

Overlapping the Kovar tube and glass collar butt joint is a metallic sleeve I8 of good conductivity, such as copper, this sleeve being shown as extending substantially from the mid-length of the tube, past the joint and to the near end of said collar. At the collar, the sleeve bends outward forming an oisetting portion I 8 and then bends again to provide a cuff portion 2U extending backward with respect to fthe sleeve portion and in spaced parallelism thereto. Said cuff portion 20 is of less length than the sleeve portion.

On the outside of the culi portion, in surface contact therewith is an enlarged cylindrical portion 2| of an outer tube which Vextends across the peripheral edge of the ring I7 and shoulders inward, at 22, next the outer face of said ring. The inwardly directed shoulder is preferably coextensive .with the flat face of the ring and at its inner periphery merges with an end tube section 23 of substantially `the diameter of the Kovar tube I.I and is concentric with and around the end portion of the rod member I3 and projects outwardly substantially as far as said rod. Near its outer end, the tube section 23 may have a coupling boss 24 secured thereon, the outer periphery of .said boss being threaded for engagement by a cooperating part of a coaxial cable or other instrumentality (not shown).

It is now appropriate to call attention to the fact that the enlarged cylindrical portion 2l extends the full length of the cuiic portion 20 and is secured thereto, the end of the enlarged cylindrical portion being spun over the end of the culi at 25 for the purpose. Spot welding, silver soldering or like modes of attachment may be substituted for or supplement 'the illustrated spun grip between the parts. Furthermore it is to be noted that end vof sleeve I8 overlapping tube II is soft soldered at its end to said tube, as at 2B, and that this soldering is done at a considerable distance from the `dielectric ring. Consequent -upon the structure described, said dielectric ring may be ofv a material of lower melting point than that of solder, a fact which is true `when said ring is made of polystyrene. Inasmuch as said sleeve, cuff and overlapping tube are all of good conducting material, such as copper, brass or the like, no appreciable electrical losses are introduced.

In process vof fabrication, the "Kovar tube, concentric rod and glass beadv are assembled on the wave generator or other device. The rest of -the connector parts may be separately assembled and then slid onto the bead and tube, and soldered thereto, as described, at 26. The ysaid dielectric ring I1 is juxtaposed to the body portion I vof the bead, both having approximately the same length. In View ofthe lower melting point of the dielectric ring, it may be melted slightly, -arter placement, by gentle torching and thus effect close contact or virtual unification between the bead and Vsaid ring.

It can be shown both theoretically and practically that the 'dimensions of .the dielectric to outer and inner peripheries thereof .have denite .relation to the limpedance and that makingthe outerdiameter of the dielectric greater, with all other factors remaining the same, the impedance is increased. By mathematical calculation it is practical to determine, therefore, proper diameter for the dielectric ring either with the ring having 4 the same (case A) or different (case B) dielectric constant from the glass bead.

Let the radius of the rod or central member I3 be designated by the letter a; the radius of the bead I5 by letter b; outer radius of the ring dielectric by letter c; and any radius between a and c by letter r.

Let e1 and e2 designate the relative dielectric constants of bead and ring, respectively, and let:

Z,=surge impedance, in ohms, of the coaxial line at the seal.

Zo=surge impedance, in ohms, of the line in vacuum preceding, and of the line in air following, the seal.

A general expression for Zl can be obtained theoretically as follows: The units to be used are in the Giorgi System `(M. K. S.-Coulomb), and

where c=velocity of electric magnetic radiation in free space.

Ilo=absolute permeability of freeV space defined as H0=41r 107 henry/meter. I0=propagation constant of line to TM (0, 1)

mode. C1, M, N--boundary-condition constants. e=distance along axis of line. V=voltage measured between conductors `of line at position z. ,azrelative permeability of medium. z`=current in amperes. 1J=velocity of propagation'of energy along line. C=capacitance per unit length of line in farad/m. :inductance per unit length of line in henry/m. ln=usua1 abbreviation for logarithm.

Case ifir-Dielectric constant of bead vand ring are equal: e1=f2=e- Inasmuch vas only the principal 'TM-mode transmits energy inthe coaxial wave guide, the electric leld strength for this mode assumes the general form:

V Inc/av J'ufst as lin electrostatics.

The charge per unit length on the inner surface of a coaxial line is, by Gauss law The velocity of energy propagation along the line is en Ve- Then the current flowing past section z of the line is meter/sec.

'L =qv=mwf amperes (7) The characteristic impedance of the line at the bead is In order that no electromagnetic energy be reflected by the `head-ring combination, it is necessary that Knowing numerical values of all quantities in Equation 12 except radius c, we can solve this equation for that numerical value, which is the dimension desired.

Case B.-Dielectric constant of bead and ring unequal; that is, e1e2- The foregoing general treatment is applicable, with modications as stated below.

Inasrnuch as the energy density is greatest in the region of maximum eld strength, which in a coaxial line is in the vicinity of the central conductor, the energy transport takes place principally through the glass bead.

This phenomenon permits the assumption that the propagation characteristics (In and v) through the bead-ring combination are, to a close approximation, the same as through the bead alone.

If all the wave energy suffers the same retardation in velocity and change in phase in traversing the seal, it retains essentially its character of a plane electromagnetic wave of the TM (0,1) mode. Then, as in case A, the transverse eld distribution is the same, as in electrostatics.

By Gauss law, as in Equation 3,

The voltage V between conductors becomes:

q l 1 2T/Alexis b/a-teahi C/b (14) The capacitance per unit length of line is 2 A C: uf, if 1 15 [an wal- 111 c/b] 61 E3 Using, as explained above,

c c 1 1 U z x-= ehu. Vg A0110 \/1 The current flowing past the seal is z=v=ai7iv (17) And the characteristic impedance of the line at the bead is In order that there be no reilection of energy at the seal,

Knowing numerical values of all quantities in Equation 20 except radius c, the equation can be solved for this desired dimension.

Thus we have provided a practical and theoretically correct means for overcoming the impedance discontinuity in a coaxial line of which one part is sealed in vacuum and an outer part is in air, by addition of a dielectric ring upon the sealing dielectric bead, and enclosing the ring Within the outer conductor. Whether the said ring has the same dielectric constant as the bead or a different constant does not alter the invention here disclosed as it is then but a matter of calculation as to the necessary diameter for the dielectric ring.

Since the various details of construction, as well as the precise relation and functioning of parts are subject to variation and change without departing from the inventive concept or scope of the invention, it is intended that all matter contained in ,fthe specification or illustrated in the drawing, shall be interpreted as exemplary and not in a limiting sense. It is also to |be understood that the following claims are intended to cover all rof the generic and specific lfeatures of the invention herein 'shown vand described and all statements of the scope of the invention herein set forth as a matter of language which might'be said to fall therebetween.

We claim:

1. A coaxial terminal assembly comprising a dielectric means, tubular coaxial line portions approaching the dielectric means at opposite ends, a central rod through the dielectric means axially disposed with respect to both said coaxial line portions, said dielectric means having one part thereof sealing one of the coaxial line portions and having a part of greater diameter than said one part, and conductive means overlying said dielectric means and connecting said tubular coaxial line portions, said dielectric means with said overlying conductive means and portion of rod therein having a surge impedance substantially equal to the surge impedance of the coaxial line portion adjacent said dielectric means.

2. A coaxial terminal assembly having a tubular portion and a coaxial rod, a bead sealing said tube portion with the rod projecting through said bead, and providing therewith a surge impedance discontinuity in the line, and means for increasing the eiTective diameter of the bead and surge impedance to substantially equal the surge impedance between :the-tubular portion and rod adjacent said bead.

3. A coaxial terminal assembly having a tubular portion and a coaxial rod, a bead sealing said tubular portion with said rod projecting through the bead, said tubular portion and bead having substantially equal outside diameters, a sleeve constituting a continuation of said tubular portion and slid thereon, a dielectric ring juxtaposed on said bead and abutting said sleeve, and a second tubular portion in axial alinement with the rst said tubular portion and abutting said ring and having an enlargement overlying said ring peripherally, thereby obtaining electrical continuity of the tubular portions and vmechanical connection therebetween.

4. A coaxial terminal assembly comprising a dielectric means, tubular coaxial line portions approaching the dielectric means at opposite ends, a central rod through the dielectric means axially disposed With respect to both said coaxial line portions, said dielectric means having a :greater 8 diameter than said tubular .coaxial line portions, and a sleeve secured to one 'of said coaxial line portions, said sleeve having a turned back cu .portion and the other-coaxial line portion having an enlarged cylindrical portion overlying said cui.

5. A coaxial terminal assembly having a tubular portion and a coaxial rod, a bead vacuum sealing said tube portion with the rod projecting through `said bead and providing therewith a surge impedance in the coaxial line, and a dielectric ycollar outside theV vacuum seal :of 'the bead increasing the elfective diameter of the fbead and surge impedance to substantially equal surge impedance between the tubular pontion and rod adjacent said bead and collar.

6. A coaxial terminal assembly comprising a pair of alined 'tubular portions and a coaxial rod, a bead vacuum sealing a iirst one of said tubular portions with said rod projecting through the bea-d and into the second Vone of said tubular portions, the rst one of said tubular portions and bead having substantially equal outside diameters, the second one of said tubular portions having an enlarged cylindrical portion overlying said bead, a dielectric ring. included between said bead and 'said enlarged cylindrical portion, and conductive means connecting the two tubular portions and having a pant thereof in sur-V face contact with the surfaces of equal diameter of said iirst tubular portion and bead.

7. A coaxial terminal assembly comprising a pair of alined tubular portions and a coaxial rod, a ibead vacuum sealing a first one of said tubular Portions with said rod projecting through the bead and into the second one of said tubular portions, the first one'of said tubular portions and bead having substantially equal outside diameters, a sleeve slipped over the adjoining parts of said bead and rst tubular portion, a dielectric ring girdling anotherv part of said bead and in endwise engagement with said sleeve, Isa-idv sleeve having a turned back cuff portion the outside diameter Whereof is substantially that of the ring, and said second vtubular portion having an enlarged portion overlying 'said ring and cuiT.

WILLIAM W. RIGROD. DAVID GORDON CLIFFORD.

Disclaimer 2,408,271.-Willfam W. Rigrod, Bloomfield, and David Gordon Oliord, Montclair, N. J. COAXIAL TERMINAL ASSEMBLY. Patent dated Sept. 24, 1946. Disclaimer filed Aug. 27, 1949, by the assignee, Westinghouse Eleotrz'o Corporation. Hereby enters this disclaimer t0 claims 1 and 2 in said specification.

[Oficial Gazette October 4, 1949.]

y. Disclaimer 2,408,271.William W." Rigrod, Bloomfield, and David Gordon Olijord, Montclair, N. J. COAXIAL TERMINAL ASSEMBLY. Patent dated Sept. 24, 1946. Disclaimer filed Aug. 27, 1949, by the assignee, Westinghouse Electric Corporation. f Hereby enters thisdisclaimer to claims 1 and 2 in said specicaton.

[O cial Gazette October 4, 1949.] 

